The present invention relates to the preparation of small spheroidal polymer beads, particularly to a method for preparing small spheroidal polymer beads having a relatively uniform particle size.
Uniformly sized polymer beads in the size range from about 5 to about 100 microns in diameter are employed in a variety of applications. Such polymer beads have been employed, for example, as calibration standards for blood cell counters, aerosol instruments, in pollution control equipment, in fast liquid protein chromatography and other chromatographic applications, as substrates for ion exchange resins, as seeds for the preparation of larger uniformly sized polymer particles, and as spacers for photographic emulsions, among other uses.
Unfortunately, however, the preparation of such uniformly sized spheroidal polymer particles using known methods is not entirely satisfactory. For example, in U.S. Pat. Nos. 4,186,120 and 4,336,173 to Ugelstad, it is taught to prepare a seed latex having oligomeric particles of low molecular weight and subsequently causing the latex particles to imbibe a material of limited water solubility. If such imbibed material is a polymerizable monomer, the monomer may be polymerized within the seed latex particle to produce uniformly sized polymer particles having diameters of up to 50 micrometers. Unfortunately, however, such process is useful only when monomers within a specific range of water solubility are employed. An additional limitation to this process is that only monomers which are amenable to an emulsion polymerization process are advantageously employed therein. Moreover, the particle size distribution is primarily controlled indirectly through the preparation of the seed latex particles. Accordingly, nonuniformly dispersed seed latex particles will give rise to product particles having a size distribution which is quite poor. Also, differences in the molecular weight or crosslinking of the polymers in the various seed latex particles as well as nonuniform imbibation of monomers thereby will give rise to nonuniformity in the sizes of the product particles.
In another known process for preparing uniform size polymer particles of about 1 to 40 micrometers, a jet of a polymer solution is vibratorily broken into droplets, and the resulting droplets are spray dried to remove the solvent therefrom. Apparatus for preparing such polymer particles according to this process are commercially available. While direct control of the size of the product particles can be exercised using this method, this method is used only for polymers which may be dissolved in some solvent. For example, this process is not useful for the preparation of uniformly sized particles of crosslinked polymers. Another problem with this method is that the aerosol generated thereby contains significant amounts of organic solvents which are evaporated from the jetted polymer solution.
Accordingly, it would be highly desirable to provide a method for preparing uniformly dispersed polymer particles having diameters in the range from about 5 to about 100 micrometers whereby the deficiencies associated with known methods can be avoided.